1. Field of the Invention
The present invention relates to a solid state image pickup device and manufacturing method therefor, and more particularly to a light receiving portion (photo diode) of a solid state image pickup device.
2. Description of the Related Art
Recently, solid state image pickup devices such as CCD devices have become widely used. FIG. 1A is a sectional view of a photo cell in a conventional solid state image pickup device. FIGS. 1B and 1C are graphical illustrations of the impurity concentration and potential distribution versus the depth from the surface, respectively. The manufacturing method for this conventional device includes the steps of forming a P type well 12 upon an N type substrate 11, forming a channel stop 13 and a vertical CCD (VCCD) 14 thereupon, forming a gate insulating layer 15, forming a charge carrying gate electrode 16, ion-implanting an N type impurity into a portion where a light receiving portion is to be formed, thereby forming a photo diode (P-D) 17. Then, P type ion impurities are implanted into the surface of the light receiving photo diode, thereby forming a P.sup.+ layer 18, and an insulating oxide layer 19 is formed thereon. Finally, a light shielding metal layer 20 and a protecting layer 20a are formed.
Thus, by forming the P.sup.+ layer 18 by ion-implantation of P type impurities into the surface of the N type photo diode 17, some problems are prevented. Specifically, thermal noise charges are prevented, and hydrogen ions, which are coupled with dangling bonds, are introduced into the photo diode 17. As a result, these noise charges are undesirably mixed with the desired signal charges. That is, the noise charges which are generated on the surface of the N type photo diode 17 are prevented from being introduced into the photo diode by the P.sup.+ layer 18 which is formed as a barrier on the surface of the photo diode 17.
Under these conditions, the P.sup.+ layer 18 can effectively serve as a barrier only if the P.sup.+ layer 18 maintains a higher impurity concentration by one or more orders of magnitude as compared with the N type photo diode 17. With such concentrations, a neutral region will exist.
FIG. 1B illustrates the impurity concentrations. Here, the concentrations are sequentially reduced in the P.sup.+, N, P, N.sup.- sequence. If the impurity concentration is distributed in this manner, and the surface potential is assumed to be zero as in FIG. 1C, the N region is a negative region, while the P region has a slightly negative potential as compared with the surface.
Thus the conventional light receiving portion has a P.sup.+ -N-P-N.sup.- structure, so that the surface charges are caught in the neutral region. If the depth of the neutral region is increased, the image pickup capability for short wavelengths is reduced, and therefore, the depth must be limited. However, if the depth is arranged to an optimum dimension, some noise charges cross the P.sup.+ layer barrier causing white noise.